The invention relates to a method for filling a vehicle fuel tank with gas by connecting the vehicle fuel tank to a delivery device with a pressure-tight line and filling the gas stored under pressure into the vehicle fuel tank from a storage unit by means of the delivery device.
Gas-fueled motor vehicles are gaining increasing importance as alternatives to conventional motor vehicles powered with liquid fuels such as gasoline or diesel fuel. The fuel typically used as the fuel for gas-fueled vehicles today is compressed natural gas. Modern developments are, however, moving to using gaseous hydrogen as a fuel.
To provide gas-fueled vehicles with a satisfactory range while simultaneously keeping the dimensions of the vehicle fuel tank within reasonable limits, the vehicle fuel tanks are normally filled to high pressures. In the case of natural gas, for example, the vehicle fuel tank is filled to an end pressure of approximately 200 bar with respect to a reference temperature of 15xc2x0 C. In the case of gaseous hydrogen, even higher end pressures are aimed for, for example 600 bar, which is primarily due to the lower energy density of hydrogen (in comparison with natural gas).
The ambient temperature, among other parameters, is taken into account during filling for a safe refueling with gas. If it is assumed, for instance, that the end temperature for natural gas should be approximately 200 bar at a reference temperature of 15xc2x0 C., when the ambient temperature is less than 15xc2x0 C., refueling must be ended at a pressure of less than 200 bar in order to ensure that an impermissibly high pressure does not occur in the vehicle fuel tank when the ambient temperature rises. Conversely, at an ambient pressure of more than 15xc2x0 C., refueling can take place up to an end pressure of more than 200 bar. The vehicle fuel tanks are therefore designed such that no impermissible pressure arises in the fuel tank even when the ambient temperature fluctuates, provided that the maximum end pressure during refueling corresponds to a pressure of 200 bar at the reference temperature of 15xc2x0 C.
A gas refueling process and a corresponding device are known, for example, from EP-A-0 653 585 which allow a very simple, and in particular also very fast, refueling of the tank comparable to refueling with gasoline. In accordance with the process proposed there, the vehicle fuel tank is connected to a delivery device via a pressure-tight line and the gas stored under pressure is filled into the vehicle fuel tank from a storage unit by means of a delivery device. First, the permissible end pressure for the instantaneously prevailing temperature is determined. Then a small quantity of gas is filled into the vehicle fuel tank in order to effect a pressure balance between the vehicle fuel tank and the delivery device. The initial pressure prevailing in the vehicle fuel tank is measured in the delivery device after the pressure balance and subsequently a specific mass of gas filled into the vehicle fuel tank. Then the mass flow of the gas is interrupted and the pressure prevailing in the vehicle fuel tank is again measured in the delivery device. A relationship is determined between the mass of the gas delivered and the pressure in the vehicle fuel tank from these pressure values. A mass of gas is calculated on the basis of this relationship such that at least one further mass of gas has to be supplied to fill the vehicle fuel tank to the end pressure. After the calculated mass of gas has been supplied to the vehicle fuel tank, the mass flow is again interrupted and the pressure prevailing in the vehicle fuel tank is determined in the delivery device. The procedures of calculating an additional mass of gas, of supplying this mass of gas and of interrupting the mass flow to determine the pressure are repeated several times until the temperature-adjusted end pressure is reached in the vehicle fuel tank.
Starting from this prior art, it is an object of the invention to provide a different process for filling a vehicle fuel tank with a gas which allows an even faster and more precise refueling without compromises in safety being required therefor. The process should also be usable for very high end pressures such as are desired in refueling with gaseous hydrogen.
A process for the filling of a vehicle fuel tank with a gas is therefore provided in accordance with the invention in which the vehicle fuel tank is connected to a delivery device via a pressure-tight line and the gas stored under pressure is filled into the vehicle fuel tank from a storage unit by means of the delivery device. The process comprises the following steps:
the instantaneous value of a parameter, which is representative of the instantaneous state of filling of the vehicle fuel tank, is determined during filling with the aid of a sensor arranged in the vehicle fuel tank;
the filling is ended when the parameter has reached a pre-determinable end value.
As the parameter, which is representative of the instantaneous state of filling of the fuel tank, is arranged in the vehicle fuel tank itself, the filling process can be carried out without interruption. The interruptions of the mass flow such as are proposed in EP-A-0-653 585 in order to determine the instantaneous respective pressure prevailing in the vehicle fuel tank in the delivery device are no longer necessary in the process of the invention because the measurement data required for the parameter are determined online in the vehicle fuel tank during the ongoing filling. In this way, the process in accordance with the invention is particularly fast and precise without compromises in safety becoming necessary. Safety is actually increased because the state of filling is determined by means of measurement data determined in the vehicle fuel tank. The detection of the data in the vehicle fuel tank allows an even more precise determination of the state of filling so that the permitted limits for filling can be utilized even more ideally, because they can be used fully without the risk of overstepping any safety limits.
The end value of the parameter at which filling is ended when it is reached is preferably independent of the temperature; that is, a parameter is selected which represents the state of filling of the vehicle fuel tank independently of the temperature. It is particularly preferred to determine the density of gas in the vehicle fuel tank as the parameter. An end value, namely the maximum permitted density or working density, can be given for the end value in the vehicle fuel tank which is independent of the respective prevailing ambient temperature during filling. The temperature-independent end value for the parameter has the advantage that it is no longer necessary to calculate a temperature-adjusted value for the end pressure of the refueling.
A further advantageous measure comprises first determining the parameter before the vehicle fuel tank is filled with gas so that the instantaneous state of filling of the vehicle fuel tank is known as a starting value prior to the start of filling. This starting value serves to compare the instantaneous state of filling determined by measurement in the vehicle fuel tank with the data determined in the delivery device during the refueling process. Discrepancies found during this comparison indicate an error and corresponding countermeasures can be initiated. As is generally the case, the mass of the gas supplied is determined by measurement in the delivery device. This mass of gas supplied is then used to determine a control value corresponding to the parameter while taking the starting value into account; the control value is compared in each case with the parameter and an error message or the aborting of filling takes place if the deviation between the parameter and the control value exceeds a pre-determinable limit value.
Operating safety can be substantially increased during gas refueling by this measure since two different determinations of the instantaneous state of filling of the vehicle fuel tank are carried out. First, the parameter representative of the instantaneous state of filling is determined using the sensor apparatus arranged in the vehicle fuel tank and, second, the control value corresponding to the parameter is determined in the delivery device using the mass of gas supplied determined by measurement there. Defects during filling, for example leaks, can be recognized reliably and at an early point by the respective comparison of these two values.
In accordance with a particularly preferred embodiment, the sensor apparatus arranged in the vehicle fuel tank comprises a temperature sensor and a pressure sensor with which the temperature and the pressure of the gas are measured in the fuel tank. The density of the gas is then determined from these in each case as the parameter.
The method in accordance with the invention is suitable for a plurality of gas refueling plants and in particular for those where compressed natural gas or gaseous hydrogen is filled into a fuel tank.